title: Mechanical stress on tensioned wires at direct and indirect loading: a biomechanical study on the Ilizarov external fixator.
abstract: BACKGROUND
  The biomechanical effect of indirect weight loading with the Ilizarov ring fixator using a weight-bearing platform has not yet been investigated. The problem of wire loosening and breakage occurs more frequently when patients are mobilised with a weight-bearing platform. Therefore, the aim of this research was to compare the influence of direct and indirect weight loading on the tensioned wires.
  
  
  METHODS
  A universal testing machine (UTS, Germany) was used in this study. A composite tibia model with a standard four-ring Ilizarov fixator and 1.8-mm wires in anatomical position was used to simulate a clinical situation. Wire strain was measured with two strain gauges positioned at the ring-wire interface of each wire. After a standardised 2-mm mid-diaphyseal osteotomy, an axial load of up to 1000 N was applied to the bone; the different methods of weight loading were evaluated in two experimental set-ups.
  
  
  RESULTS
  A higher axial load was necessary to achieve an osteotomy gap closure at indirect loading. Mechanical stress on the tensioned wires was 400% higher on the proximal wires and 250% higher on the distal wires at a maximum axial loading of 1000 N. Mechanical stress remained on the wires in indirect loading, even after bone end contact, and led to excessive stress under higher weight-bearing amounts.
  
  
  CONCLUSIONS
  There is a substantial change in the biomechanical characteristics of the Ilizarov ring fixator when mobilising a patient with a weight-bearing platform. The considerable higher mechanical stress on the wires needs to be considered when patients are mobilised with a weight-bearing platform.
authors.affiliation: BG Universitätsklinikum Bergmannsheil, Department of Traumatology, Bürkle-de-la-Camp-Platz 1, 44789 Bochum, Germany. jangessmann@gmx.de
authors.forename: Jan
authors.initials: J
authors.lastname: Gessmann
authors.forename: Birger
authors.initials: B
authors.lastname: Jettkant
authors.forename: Thomas Armin
authors.initials: TA
authors.lastname: Schildhauer
authors.forename: Dominik
authors.initials: D
authors.lastname: Seybold
mesh_terms: D001696:Biomechanical Phenomena; D001864:Bone Wires; D004867:Equipment Design; D019544:Equipment Failure Analysis; D016267:External Fixators; D018460:Fractures, Comminuted / Q000601:surgery; D006801:Humans; D018889:Ilizarov Technique / Q000295:instrumentation*; D008422:Materials Testing / Q000379:methods; D013314:Stress, Mechanical; D013718:Tensile Strength; D013978:Tibial Fractures / Q000601:surgery; D016474:Weight-Bearing